1. Field of the Invention
This invention relates to an improved process for manufacturing vesiculated beads. The beads of this invention have special utility as opacifying agents and show improved scattering efficiency and resistance to shrinkage upon drying.
2. Description of the Prior Art
The technique of preparing porous articles from carboxylated, unsaturated polyester resins by emulsifying water into the polyester resin in the presence of a base and then polymerizing the resin has been known in the prior art. For example, U.S. Pat. No. 3,255,137 teaches the preparation of porous polymeric materials by a process involving dispersing water in a polymerizable liquid, dispersing the resulting emulsion in water and subsequently polymerizing the liquid.
U.S. Pat. No. 3,822,224 teaches a process of preparing vesiculated polyester resin granules by dispersing a styrene solution of carboxylated unsaturated polyester resin in an aqueous continuous phase in the presence of a base. In that process, the vesicles form spontaneously within the granules. Pigment could be included within the granules by a double emulsion process which involves the dispersion of pigment in water, the dispersion of the pigment dispersion into the polyester solution to give an emulsion of pigment dispersion in polymer, and the dispersion of this emulsion into the base-containing aqueous phase. These vesiculated beads were useful, but the granules shrank upon drying which could cause cracking and other defects in films containing the vesiculated beads as opacifiers.
U.S. Pat. No. 3,879,314 taught that dimensionally stable beads could be prepared by selecting a polyester resin having an acid value of 10-45 and utilizing an amine having a specified pKa range at a concentration such that there were 0.3 to 1.4 amine groups present per polyester resin carboxyl group.
Suprisingly, it has now been found that dimensionally stable vesiculated beads can be prepared by using amine concentrations outside of the range taught in U.S. Pat. No. 3,879,314 and that these novel beads have improved scattering efficiency and therefore act as more efficient opacifiers then the beads of the prior art.